This application claims the priority of German Application No. 198 38 223.5, filed Aug. 22, 1998, the disclosure of which is expressly incorporated by reference herein. The present invention relates to a method for determining the ion component following a combustion process in a self-igniting internal combustion engine.
A method for determining the ion component is already known from WO 86/00961, in which the ion current in a diesel engine is measured using a DC voltage. For this purpose, a modified glow plug is introduced into the combustion chamber of the cylinder in which an ion current measurement is to be performed. On its tip projecting into the combustion chamber, the modified glow plug has an electrically conducting layer connected to an electrical printed conductor that terminates in an external connection for electrical contact. The ion current is measured by charging the tip of the glow plug to a DC potential of 250 volts relative to the ground potential of the combustion chamber wall. Therefore, the tip of the glow plug forms one electrode while the combustion chamber wall, which is at ground potential, forms the other electrode.
On the other hand, the goal of the present invention is to propose a method for determining the ion component following a combustion process in a self-igniting internal combustion engine, by which the measurement of the ion current in a self-igniting internal combustion engine is improved upon.
This goal is achieved according to the invention by a method for determining the ion component following a combustion process in a self-igniting internal combustion engine, in which two electrodes are located inside at least one cylinder, to which electrodes an electrical voltage can be applied, characterized in that the voltage is an AC voltage.
Applicants"" previously filed German patent application (filed May 16, 1997 at the German Patent Office and with official file no. P . . . ) performs an ion current measurement in a four-cycle engine using a spark plug as a measurement probe, with this spark plug being charged with an alternating voltage. Since in that case the noise signal is evaluated by a measuring resistor, the ion current signal can be determined. It has been found that a carbon coating forms over the spark plug which can lead to a shunt resistance between the electrodes of the spark plug. When an ion current measurement is to be performed, this shunt resistance has a disturbing influence since, when a DC voltage on the order of several hundred volts is applied as a measuring voltage, the current that flows through this shunt resistance can be of the same order of magnitude as the ion current to be evaluated.
It has been found that the field strength or the polarity of the voltage on the spark plug is not symmetrical. With negative polarity of the glow plug, a much weaker intensity develops than when the polarity of the glow plug is positive. As a result, a measurement signal is obtained in which the input signal is modulated by the ion current that flows due to electrical fields of different magnitudes that form as a function of the polarity of the glow plug. Only an ohmic resistance is formed by the carbon layer, by which the measurement signal is not modulated.
This goal is also achieved by another embodiment of the method according to the invention, in which the applied voltage has an AC component. The operation in this method is explained in the same way as in the first embodiment. The AC component is modulated by the ion current.
In a further advantageous method according to the invention, the measurement signal is evaluated as a voltage across a measuring resistance. This provides a very simple way of detecting the measurement signal.
In another preferred embodiment of the method according to the invention, the ion current signal is obtained with the voltage across the measuring resistance being subjected to low-pass filtration.
It turns out that because of the modulation of the measured signal, caused by the ion current, a comparatively high power density in the ion current signal to be evaluated is obtained at lower frequencies. This signal component can be evaluated by low-pass filtration of the measured signal.
In yet another embodiment of the method according to the invention, the ion component is obtained from the evaluation of the measured capacitance between the electrodes.
The change in dielectric constant is evaluated. The change in the dielectric constant between the electrodes is obtained from the ion density. Therefore, the dielectric constant can be determined from a measurement of the capacitance. Once again, the ion density can be derived from the dielectric constant.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.